Many buildings are located on contaminated ground. The contamination may be a toxic volatile substance, such as a volatile organic compound (VOC), or a radioactive gas, such as radon. These toxic volatile substances must be kept out of a building to maintain the health of the people within the building. One way to reduce the incursion of toxic vapors is to reduce the pressure in the ground below the building so that air within the building flows down into the ground. Thus vapors in the ground cannot flow into the building. Many buildings have a concrete slab or other relatively impermeable layer as their lower boundary with the ground. This relatively impermeable layer is called a “sub slab”. The pressure below the sub slab can be reduced by using an exhaust blower. The blower sucks air from underneath the sub slab and exhausts it away from the building. Blowers require power. There is need, therefore, for a blower control system that minimizes power without allowing ground vapors into a building.